CSF stands for "community supported fishery." It is based on CSA (community supported agriculture) in which member consumers buy a "share" of a farm and its produce and other products.
In CSFs members buy a share of a fisherman's or fishery's catch. The fishermen get guaranteed retail sales and the members get über-fresh fish.
Most CSFs operate for an entire season, doing weekly deliveries or pick-up sites. Some offer different size shares. Be sure to ask how much fish you can expect and if it will be cleaned (and to what extent) before you join!